An X-ray detector can be realized as a flat radiation detector based on solid-state imaging elements such as active matrix, CCD, and CMOS. Such an X-ray detector is drawing attention as a new-generation X-ray image detector for diagnosis. A radiographic image or real-time X-ray image is outputted as digital signals by irradiating this X-ray detector with X-rays.
The X-ray detector includes a photoelectric conversion substrate for converting light to electrical signals, and a scintillator layer in contact with the photoelectric conversion substrate. The scintillator layer converts externally incident X-rays to light. The light converted from incident X-rays in the scintillator layer reaches the photoelectric conversion substrate and is converted to electric charge. This charge is read as an output signal and converted to digital image signals in e.g. a prescribed signal processing circuit.
The scintillator layer may be made of CsI, which is a halide. In this case, incident X-rays cannot be converted to visible light by CsI alone. Thus, as in commonly-used phosphors, an activator is contained to activate excitation of light in response to incident X-rays.
In the X-ray detector, the light reception sensitivity of the photoelectric conversion substrate has a peak wavelength around 400-700 nm in the visible range. Thus, in the case where the scintillator layer is made of CsI, Tl is used as an activator. Then, the light excited by incident X-rays has a wavelength around 550 nm.
The scintillator layer may be made of a phosphor containing Tl as an activator in CsI, which is a halide. In this case, as in commonly-used phosphors containing an activator, the characteristics of the scintillator layer are significantly affected by the concentration and concentration distribution of Tl serving as an activator.
In the X-ray detector including a scintillator layer containing an activator, lack of optimization of the concentration and concentration distribution of the activator incurs characteristics degradation of the scintillator layer. This affects the sensitivity (light emission efficiency) and residual image (the phenomenon in which the subject image of the X-ray image at the (n−1)-th or earlier time remains in the X-ray image at the n-th time) related to the light emission characteristics of the scintillator layer.
For instance, in diagnosis using X-ray images, the radiography condition significantly varies with subjects (incident X-rays at a dose of approximately 0.0087-0.87 mGy, because the X-ray transmittance varies with body regions). This may cause a significant difference in the dose of incident X-rays between the (n−1)-th X-ray image and the n-th X-ray image. Here, if the dose of incident X-rays in the (n−1)-th X-ray image is greater than that in the n-th X-ray image, the light emission characteristics of the scintillator layer in the non-subject part of the (n−1)-th X-ray image is changed by the great energy of incident X-rays. This influence remains also in the n-th X-ray image and produces a residual image.
In diagnosis using X-ray images, the residual image characteristic is more important than other characteristics of the scintillator layer such as sensitivity (light emission efficiency) and resolution (MTF).
Conventionally, there have been proposals for defining the concentration and concentration distribution of the activator of the scintillator layer for the purpose of improving sensitivity (light emission efficiency) and resolution (MTF).